69 research outputs found
Stability and magnetic properties of Fe double-layers on Ir (111)
We investigate the interplay between the structural reconstruction and the
magnetic properties of Fe doublelayers on Ir (111)-substrate using
first-principles calculations based on density functional theory and mapping of
the total energies on an atomistic spin model. We show that, if a second Fe
monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal
close-packed to bcc (110)-like accompanied by a reduction of symmetry from
trigonal to centered rectangular. Although the bcc-like surface has a lower
coordination, we find that this is the structural ground state. This
reconstruction has a major impact on the magnetic structure. We investigate in
detail the changes in the magnetic exchange interaction, the magnetocrystalline
anisotropy, and the Dzyaloshinskii Moriya interaction depending on the stacking
sequence of the Fe double-layer. Based on our findings, we suggest a new
technique to engineer Dzyaloshinskii Moriya interactions in multilayer systems
employing symmetry considerations. The resulting anisotropic
Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or
antiskyrmions
Magnetic Interactions in BiFeO: a First-Principles Study
First-principles calculations, in combination with the four-state energy
mapping method, are performed to extract the magnetic interaction parameters of
multiferroic BiFeO. Such parameters include the symmetric exchange (SE)
couplings and the Dzyaloshinskii-Moriya (DM) interactions up to second nearest
neighbors, as well as the single ion anisotropy (SIA). All magnetic parameters
are obtained not only for the structural ground state, but also for the
and phases in order to determine the effects of
ferroelectricity and antiferrodistortion distortions, respectively, on these
magnetic parameters. In particular, two different second-nearest neighbor
couplings are identified and their origins are discussed in details. Moreover,
Monte-Carlo (MC) simulations using a magnetic Hamiltonian incorporating these
first-principles-derived interaction parameters are further performed. They
result (i) not only in the accurate prediction of the spin-canted G-type
antiferromagnetic structure and of the known magnetic cycloid propagating along
a direction, as well as their unusual characteristics (such
as a weak magnetization and spin-density-waves, respectively); (ii) but also in
the finding of another cycloidal state of low-energy and that awaits to be
experimentally confirmed. Turning on and off the different magnetic interaction
parameters in the MC simulations also reveal the precise role of each of them
on magnetism
Dzyaloshinskii-Moryia interaction at an antiferromagnetic interface: first-principles study of FeIr bilayers on Rh(001)
We study the magnetic interactions in atomic layers of Fe and 5d
transition-metals such as Os, Ir, and Pt on the (001) surface of Rh using
first-principles calculations based on density functional theory. For both
stackings of the 5d-Fe bilayer on Rh(001) we observe a transition from an
antiferromagnetic to a ferromagnetic nearest-neighbor exchange interaction upon
5d band filling. In the sandwich structure 5d/Fe/Rh(001) the nearest neighbor
exchange is significantly reduced. For FeIr bilayers on Rh(001) we consider
spin spiral states in order to determine exchange constants beyond nearest
neighbors. By including spin-orbit coupling we obtain the Dzyaloshinskii-Moriya
interaction (DMI). The magnetic interactions in Fe/Ir/Rh(001) are similar to
those of Fe/Ir(001) for which an atomic scale spin lattice has been predicted.
However, small deviations between both systems remain due to the different
lattice constants and the Rh vs. Ir surface layers. This leads to slightly
different exchange constants and DMI and the easy magnetization direction
switches from out-of-plane for Fe/Ir(001) to in-plane for Fe/Ir/Rh(001).
Therefore a fine tuning of magnetic interactions is possible by using single 5d
transition-metal layers which may allow to tailor antiferromagnetic skyrmions
in this type of ultrathin films. In the sandwich structure Ir/Fe/Rh(001) we
find a strong exchange frustration due to strong hybridization of the Fe layer
with both Ir and Rh which drastically reduces the nearest-neighbor exchange.
The energy contribution from the DMI becomes extremely large and DMI beyond
nearest neighbors cannot be neglected. We attribute the large DMI to the low
coordination of the Ir layer at the surface. We demonstrate that higher- order
exchange interactions are significant in both systems which may be crucial for
the magnetic ground state
Mechanism for the {\alpha} -> {\epsilon} phase transition in iron
The mechanism of the {\alpha}-{\epsilon} transition in iron is reconsidered.
A path in the Burgers description of the bcc/hcp transition different from
those previously considered is proposed. It relies on the assumption that shear
and shuffle are decoupled and requires some peculiar magnetic order, different
from that of {\alpha} and {\epsilon} phases as found in Density-Functional
Theory. Finally, we put forward an original mechanism for this transition,
based on successive shuffle motion of layers, which is akin to a
nucleation-propagation process rather than to some uniform motion.Comment: 6 pages, 5 figure
Revealing the correlation between real-space structure and chiral magnetic order at the atomic scale
We image simultaneously the geometric, electronic and magnetic structure of a
buckled iron bilayer film that exhibits chiral magnetic order. We achieve this
by combining spin-polarized scanning tunneling microscopy and magnetic exchange
force microscopy (SPEX), to independently characterize the geometric as well as
the electronic and magnetic structure of non-flat surfaces. This new SPEX
imaging technique reveals the geometric height corrugation of the
reconstruction lines resulting from strong strain relaxation in the bilayer,
enabling the decomposition of the real-space from the eletronic structure at
the atomic level, and the correlation with the resultant spin spiral ground
state. By additionally utilizing adatom manipulation, we reveal the chiral
magnetic ground state of portions of the unit cell that were not previously
imaged with SP-STM alone. Using density functional theory (DFT), we investigate
the structural and electronic properties of the reconstructed bilayer and
identify the favorable stoichiometry regime in agreement with our experimental
result
B-T phase diagram of Pd/Fe/Ir(111) computed with parallel tempering Monte Carlo
We use an atomistic spin model derived from density functional theory
calculations for the ultra-thin film Pd/Fe/Ir(111) to show that temperature
induces coexisting non-zero skyrmion and antiskyrmion densities. We apply the
parallel tempering Monte Carlo method in order to reliably compute
thermodynamical quantities and the B-T phase diagram in the presence of
frustrated exchange interactions. We evaluate the critical temperatures using
the topological susceptibility. We show that the critical temperatures depend
on the magnetic field in contrast to previous work. In total, we identify five
phases: spin spiral, skyrmion lattice, ferromagnetic phase, intermediate region
with finite topological charge and paramagnetic phase. To explore the effect of
frustrated exchange interactions, we calculate the B-T phase diagram, when only
effective exchange parameters are taken into account.Comment: 8 figure
Engineering magnetic domain wall energies in multiferroic BiFeO via epitaxial strain
Epitaxial strain has emerged as a powerful tool to tune magnetic and
ferroelectric properties in functional materials such as in multiferroic
perovskite oxides. Here, we use first-principles calculations to explore the
evolution of magnetic interactions in the antiferromagnetic multiferroic
BiFeO (BFO), one of the most promising multiferroics for future technology.
The epitaxial strain in BFO(001) oriented film is varied between
. We find that both strengths of the
exchange interaction and Dzyaloshinskii-Moriya interaction (DMI) decrease
linearly from compressive to tensile strain whereas the uniaxial
magnetocrystalline anisotropy follows a parabolic behavior which lifts the
energy degeneracy of the (111) easy plane of bulk BFO. From the trends of the
magnetic interactions we can explain the destruction of cycloidal order in
compressive strain as observed in experiments due to the increasing anisotropy
energy. For tensile strain, we predict that the ground state remains unchanged
as a function of strain. By using the domain wall (DW) energy, we envision the
region where isolated chiral magnetic texture might occur as function of strain
i.e. where the DW and the spin spiral energy are equal. This transition between
and of strain should allow topologically stable magnetic
states such as antiferromagnetic skyrmions and merons to occur. Hence, our work
should trigger experimental and theoretical investigations in this range of
strain
Trochoidal motion and pair generation in skyrmion and antiskyrmion dynamics under spin-orbit torques
Skyrmions and antiskyrmions in magnetic ultrathin films are characterised by
a topological charge describing how the spins wind around their core. This
topology governs their response to forces in the rigid core limit. However,
when internal core excitations are relevant, the dynamics become far richer. We
show that current-induced spin-orbit torques can lead to phenomena such as
trochoidal motion and skyrmion-antiskyrmion pair generation that only occurs
for either the skyrmion or antiskyrmion, depending on the symmetry of the
underlying Dzyaloshinskii-Moriya interaction. Such dynamics are induced by core
deformations, leading to a time-dependent helicity that governs the motion of
the skyrmion and antiskyrmion core. We compute the dynamical phase diagram
through a combination of atomistic spin simulations, reduced-variable
modelling, and machine learning algorithms. It predicts how spin-orbit torques
can control the type of motion and the possibility to generate skyrmion
lattices by antiskyrmion seeding
Spin-current driven Dzyaloshinskii-Moriya interaction in the multiferroic BiFeO3 from first-principles
The electrical control of magnons opens up new ways to transport and process
information for logic devices. In magnetoelectrical multiferroics, the
Dzyaloshinskii-Moriya (DM) interaction directly allow for such a control and,
hence, is of major importance. We determine the origin and the strength of the
(converse) spin current DM interaction in the R3c bulk phase of the
multiferroic BiFeO3 based on density functional theory. Our data supports only
the existence of one DM interaction contribution originating from the spin
current model. By exploring then magnon dispersion in the full Brillouin Zone,
we show that the exchange is isotropic, but the DM interaction and anisotropy
prefer any propagation and any magnetization direction within the full (111)
plane. Our work emphasizes the significance of the asymmetric potential induced
by the spin current over the structural asymmetry induced by the anionic
octahedron in multiferroics such as BiFeO3
Comparison of first-principles methods to extract magnetic parameters in ultra-thin films: Co/Pt(111)
We compare three distinct computational approaches based on first-principles
calculations within density functional theory to explore the magnetic exchange
and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111),
namely (i) the method of infinitesimal rotations of magnetic moments based on
the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized
Bloch theorem applied to spiraling magnetic structures and (iii) supercell
calculations with non-collinear magnetic moments, the latter two being based on
the full-potential linearized augmented plane wave (FLAPW) method. In
particular, we show that the magnetic interaction parameters entering
micromagnetic models describing the long-wavelength deviations from the
ferromagnetic state might be different from those calculated for fast rotating
magnetic structures, as they are obtained by using (necessarily rather small)
supercell or large spin-spiral wave-vectors. In the micromagnetic limit, which
we motivate to use by an analysis of the Fourier components of the domain-wall
profile, we obtain consistent results for the spin stiffness and DMI
spiralization using methods (i) and (ii). The calculated spin stiffness and
Curie temperature determined by subsequent Monte Carlo simulations are
considerably higher than estimated from the bulk properties of Co, a
consequence of a significantly increased nearest-neighbor exchange interaction
in the Co-monolayer (+50%). The calculated results are carefully compared with
the literature
- …